internal/trace/v2: clean up the ordering interface
This change cleans up the ordering interface in several ways.
First, it resolves a TODO about using a proper queue of events for extra
events produced while performing ordering. This will be necessary for a
follow-up change to handle coroutine switch events.
Next, it simplifies the ordering.advance method's signature by not
returning a schedCtx. Instead, ordering.advance will take responsibility
for constructing the final Event instead of the caller, and places it on
its own internal queue (in addition to any other Events generated). The
caller is then responsible for taking events off of the queue with a new
method Next.
Finally, hand-in-hand with the signature change, the implementation of
ordering.advance no longer forces each switch case to return but instead
has them converge past the switch. This has two effects. One is that we
eliminate the deferred call to update the M state. Using a defer here is
technically incorrect, because we might end up changing the M state even
if we don't advance the event! We got lucky here that curCtx == newCtx
in all such cases, but there may have been a subtle bug lurking here.
Unfortunately because of the queue's semantics however, we can't
actually avoid pushing into the queue at every possible successful exit
out of the switch. Hopefully this can become less error-prone in the
future by splitting up the switch into a dispatch of different
functions, instead of everything living in one giant function. This
cleanup will happen in a follow-up change.
Change-Id: Ifebbbf14e8ed5c08be5c1b0fadc2e5df3915c656
Reviewed-on: https://go-review.googlesource.com/c/go/+/565936
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>